It is known that, of the numerous processing steps involved in the manufacture of semiconductor integrated circuits (e.g. diffusion, epitaxial growth, metallurgy, etc.), the lithographic steps are the most important, not only because they are frequently performed (some 10 masking steps are required for a MOSFET, while approximately 15 are used for a bipolar transistor), but also because they ultimately determine the density of the circuits. The efforts made to achieve an improved density have resulted in a significant development of lithographic techniques, which now involve electronbeam and x-ray exposures rather than the UV radiation exposures formerly used. Although finer and finer resolutions have thus been obtained (0.5 to 1 micron), the lithographic steps have become more critical. It is well known that the masking layers (or the masks) used in the manufacture of integrated circuits vary from the specifications established by the designer, as a result of undesirable effects such as overexposure or underexposure of the photoresist layers which may occur during fabrication of the masking layers or masks. Also, even if the width of the masking layer (or mask) is equivalent to the desired nominal width (W) of a line, any overetching or underetching of the insulating layer will result in the diffused regions being too wide or too narrow as compared with the nominal value. Lines exhibiting significant width variations (.DELTA.W) relative to the nominal (design) value (W) can pose reliability problems due to short circuits or to open circuits and must therefore be detected as early as possible during the manufacturing process.
Another important characteristic is that the diffused regions (resistors, lines, capacitors, etc.) formed through an insulating layer etched in accordance with a desired pattern and which may exhibit dimensional variations, can result in the components having incorrect ratings and thus create problems during operation.
It is therefore important to correct misalignments and other defects that will cause a shift in the position of the image. These shifts are expressed in terms of dimensional variations: instead of an etched area having a desired width W, there is obtained an etched area of which .DELTA.W+W (.DELTA.W being positive or negative). The relative tolerance is defined by the ratio .DELTA.W/W. Heretofore, this was determined visually, a time-consuming, costly process which did not yield meaningful data bases.
Also, it is often useful to know the overlay tolerance applicable to a couple of superimposed images. Usually, this tolerance is measured and checked visually, which entails the same disadvantages as those mentioned above.